The code ends up slightly longer, but the decoding is more consistent from
one instruction class to another, so hopefully it's easier to make sense of it.

Change-Id: Ia22c2ebb0865536da0c3dac6876bdb0b20075f04
Reviewed-on: https://go-review.googlesource.com/21215Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>